1. Technical Field
The present invention relates to an in-plane switching mode liquid crystal display panel, and more particularly to a fringe field switching (hereinafter, referred to as “FFS”) mode liquid crystal display panel achieving a large aperture ratio, bright display, and a good characteristic obtained upon applying a force to a surface.
2. Related Art
Three electric field modes, that is, a TN (Twisted Nematic) mode, a VA (Vertical Alignment) mode, and an MVA (Multi-domain Vertical Alignment) mode are mainly used in a liquid crystal panel. An operation principle of the FFS mode liquid crystal display panel of the in-plane switching mode liquid crystal display panel in which electrodes are formed only in one substrate will be described with reference to FIGS. 11 and 12 (see JP-A-2002-14363 and JP-A-2002-244158).
FIG. 11 is a schematic plan view illustrating one pixel in a color filter substrate of the FFS mode liquid crystal display panel according to a known example. FIG. 12 is a sectional view taken along the line XII-XII of FIG. 11.
An FFS mode liquid crystal display panel 70 includes an array substrate AR and a color filter substrate CF. In the array substrate AR, a plurality of scanning lines 72 and a plurality of common lines 73 are arranged on the surface of a first transparent substrate 71 so as to be parallel to each other and a plurality of signal lines 74 are arranged so as to intersect the scanning lines 72 and the common lines 73. Counter electrodes 75 formed of a transparent conductive material such as ITO (Indium Tin Oxide) or IZO (Indium Zinc Oxide) and connected to the common lines 73 are disposed so as to cover areas defined by the scanning lines 72 and the signal lines 74. Each of pixel electrodes 78 formed of a transparent conductive material such as ITO and having a plurality of slits 77 is formed so as to have a stripe arrangement on the surface counter electrode 75 through an insulation film 76. The surfaces of the pixel electrode 78 and the plurality of slits 77 are covered with an alignment film 80.
TFTs which are switching elements are formed in the vicinity of locations where the scanning lines 72 and the signal lines 74 intersect each other. In each of the TFTs, a semiconductor layer 79 is disposed on the surface of the scanning line 72, a source electrode S is formed by extending a part of the signal line 74 so as to cover a part of the surface of the semiconductor layer 79, a portion below the semiconductor 79 forms a gate electrode G, a conductive layer overlapping with a part of the semiconductor layer 79 forms a drain electrode D, and the drain electrode D is connected to the pixel electrode 78.
The color filter substrate CF includes a color filter layer 83, an overcoat layer 84, and an alignment film 85 on the surface of a second transparent substrate 82. The array substrate AR and the color filter substrate CF are opposed to each other so that the pixel electrodes 78 and the counter electrodes 75 of the array substrate AR are opposed to the color filter layer 83 of the color filter substrate CF. Liquid crystal LC is sealed between the array substrate AR and the color filter substrate CF and polarizing plates 86 and 87 are disposed on the outside of both the array substrate AR and the color filter substrate CF, respectively, so that polarizing directions are perpendicular to each other. In this way, the FFS mode liquid crystal display panel 70 is formed.
In the FFS mode liquid crystal display panel 70, when an electric field is generated between each of the pixel electrodes 78 and each of the counter electrodes 75, the electric field is oriented from both sides of the pixel electrode 78 toward the counter electrode 75, as shown in FIG. 11. With such a configuration, not only the liquid crystal present in the slits 77 but also the liquid crystal present in each of the pixel electrodes 78 can be moved. Accordingly, the FFS mode liquid crystal display panel 70 is capable of achieving a wider viewing angle, a higher contrast, and a higher transmissivity than those of a known IPS mode liquid crystal display panel, thereby realizing bright display. Moreover, the FFS mode liquid crystal display panel 70 has an advantage of obtaining larger storage capacitance and thus not requiring an additional capacitor line, since an area where the pixel electrode 78 and the counter electrode 75 overlap with each other in plan view is larger than that in the IPS mode liquid crystal display panel.
However, in the FFS mode liquid crystal display panel and the IPS mode liquid crystal display panel, a reverse twist domain occurs in electrode ends. In addition, it is known that the area in which the reverse twist domain occurs spreads out in a test in which a force is applied to a surface and thus a ripple problem or the like occurs. A principle of the occurrence of the reverse twist domain will be described with reference to FIGS. 13A and 13B and FIGS. 14A to 14E.
FIG. 13A is a schematic expanded plan view illustrating an end of each slit-shaped opening of an upper electrode in the FFS mode liquid crystal display panel with no application of voltage. FIG. 13B is a schematic expanded plan view illustrating the end thereof with application of voltage. FIGS. 14A to 14E are diagrams illustrating an angle φ formed between a boundary direction of each slit-shaped opening and a rubbing direction Y.
In an upper electrode 91, a slit-shaped opening 92 is formed so as to be tilted in an oblique direction, for example. An insulation film is present below the slit-shaped opening 92 and a lower electrode is present below the insulation film. Under the assumption that the rubbing direction with respect to the slit-shaped opening 92 is a Y direction in FIG. 13A, liquid crystal molecules 93a are oriented in the rubbing direction with no application of voltage. When driving voltage is applied between the upper electrode 91 and the lower electrode, an electric field E is generated in a direction perpendicular to the boundary of the slit-shaped opening 92 between the upper electrode 91 and the lower electrode and the liquid crystal molecules 93a horizontally rotate by a predetermined angle θ, for example, in correspondence to the electric field E, as shown in FIG. 13B.
In this case, the directions of the electric field E on long sides 94a and 94b of the slit-shaped opening 92 are the same. However, at an end 95 of the slit-shaped opening 92, the direction of the electric field F gradually varies by 180° between one long side 94a and the other long side 94b of the slit-shaped opening 92. Therefore, an abnormal alignment area where the alignment direction of the liquid crystal molecules can rotate either rightward or leftward is present at the end 95 of the slit-shaped opening 92, when a driving voltage is applied between the upper electrode 91 and the lower electrode.
That is, as shown in FIG. 14A, under the assumption that the rubbing direction of the alignment film is Y, a longitudinal direction of the slit-shaped opening 92 of the upper electrode 91 is Z, and an angle direction of an acute angle oriented from Z to Y is positive, an acute angle φ (where φ>0°) formed between the boundary direction of the slit-shaped opening and rubbing direction Y is positive at a location A of the slit-shaped opening. The boundary direction of the slit-shaped opening and the rubbing direction Y are parallel to each other at a location B of the slit-shaped opening shown in FIG. 14B, so that φ=0°. At a location C of the slit-shaped opening shown in FIG. 14C, a direction of the acute angle φ oriented from the location C of the slit-shaped opening to the rubbing direction Y is reverse to the direction of the acute angle φ at the location A shown in FIG. 14A, so that the acute angle φ becomes negative (where φ<0°). Likewise, the acute angle φ becomes φ=±90° at a location D of the slit-shaped opening D shown in FIG. 14D. In addition, at a location E of the slit-shaped opening shown in FIG. 14E, the acute angle φ formed between the boundary direction of the slit-shaped opening and the rubbing direction Y is oriented in the same direction as that of the acute angle at the location A shown in FIG. 14A, so that the acute angle φ is positive (where φ>0°).
Accordingly, the end 95 of the slit-shaped opening 92 includes the abnormal alignment area where the alignment direction of the liquid crystal molecules can rotate either rightward or leftward between the location B shown in FIG. 14B and the location D shown in FIG. 14D, when the driving voltage is applied between the upper electrode 91 and the lower electrode. This abnormal alignment area of the end 95 of the slit-shaped opening 92 is called the reverse twist domain. In the known FFS mode liquid crystal display panel, since an image cannot be displayed in the reverse twist domain, the reverse twist domain is generally shielded by a light-shielding member. However, when the reverse twist domain is shielded by the light-shielding member, a decrease in aperture ratio occurs.